Publications by Thorsten Hesjedal


Fabrication of Microfluidic Components Actuated by Thermoresponsive Hydrogels

Book of Ab­stracts (2003)

ME Harmon, T Hesjedal, M Tang, D Kuckling, CW Frank


Calculation and Experimental Verification of the Acoustic Stress at GHz Frequencies in Resonators

Proc. of the Workshop on Piezoelectric Resonators for Sensor Applications (2003)

F Kubat, W Ruile, L Reindl, T Hesjedal


Near-field elastomeric mask photolithography fabrication of high-frequency surface acoustic wave transducers

Nanotechnology 14 (2003) 91-94

T Hesjedal, W Seidel

Optical lithography is the method of choice for mass production of electronic as well as acoustic devices. Cost issues, in particular, make it superior over slow but high-resolution methods, such as electron beam lithography. Also, its applicability for nonconductive substrates is an important feature for acoustic device fabrication on ceramics. In order to be able to continue the use of diffraction-limited optical lithography, new schemes have been developed that enhance the resolution. Rather complex phase-shifting masks, for instance, alter both the amplitude and the phase of the exposing light and lead to higher resolution. However, by using an elastomeric phase mask derived from a photoresist master (made by conventional photolithography), features as small as 90 nm have been demonstrated. We report on the application of the near-field phase shift technique for the fabrication of surface acoustic wave (SAW) devices. This technique is best suited for the fabrication of narrow electrode gap SAW devices that are designed for the efficient SAW excitation at higher harmonics. The combination of near-field phase shift lithography with narrow-gap SAW designs thus opens up a way for simple and low-cost SAW devices operating above 5 GHz.


Near-field phase shift photolithography for high-frequency SAW transducers

Proceedings of the IEEE Ultrasonics Symposium 1 (2002) 247-250

T Hesjedal, W Seidel, H Kostial

Optical lithography has been widely used in mass production of various electronic devices, mainly because of its high throughput capability. However, the resolution in conventional lithography is diffraction limited. Cost issues, on the other hand, make slower but higher resolution methods, like electron beam lithography, unattractive for industrial applications. In order to be able to continue the use of optical lithography, new schemes were developed that enhance the resolution. Phase-shifting masks, for example, alter both the amplitude and the phase of the exposing light and lead to higher resolution. Using the related phase edge method it has been shown that 100 nm features can be produced using 248 nm light. Furthermore, employing an elastomeric phase mask, commercially available photo resist, and incoherent light, down to 90 nm features were demonstrated. Here, we report on the application of a near-field phase shift technique on the fabrication of SAW transducers. This simple and low cost technique is best suited for the fabrication of SAW structures, where the metallization ratio is different from 1:1, like Narrow Gap Floating Electrode Unidirectional Transducers (NG-FEUDTS).


Study of surface acoustic wave scattering and diffraction by scanning acoustic force microscopy

Proceedings of the IEEE Ultrasonics Symposium 1 (2002) 581-584

T Hesjedal, W Seidel

Scanning Acoustic Force Microscopy (SAFM) has been used to study scattering and diffraction phenomena of SAWs (surface acoustic waves) with a spatial resolution on the nanoscale. With other acoustic imaging methods, it has been difficult to achieve both nanometer lateral resolution and sub-Å wave amplitude detection sensitivity at the same time. With SAFM, amplitude distribution and phase velocity of arbitrarily polarized modes are accessible on the nanoscale. We present a model of the SAFM contrast formation and give some examples for simple experimental situations. SAW excitation by a single SAW source and the wavefield distribution within an acoustic charge transport structure are examined.


The origin of ultrasound-induced friction reduction in microscopic mechanical contacts.

IEEE Trans Ultrason Ferroelectr Freq Control 49 (2002) 356-364

T Hesjedal, G Behme

We present a study of the origin of ultrasound-induced friction reduction in microscopic mechanical contacts. The effect of friction reduction caused by Rayleigh-type surface acoustic waves (SAWs) is demonstrated for propagating and two-dimensional, standing wave fields using lateral force microscopy (LFM). It is shown that with increasing wave amplitude, friction is completely suppressed. To detect and distinguish between the effect of lateral and vertical surface oscillation components on the cantilever movement, we employed multimode scanning acoustic force microscopy (SAFM). We found that the friction reduction effect is only due to the vertical oscillation component. Because this effect does not appear for purely in-plane polarized Love waves, we concluded that the mechanical diode effect is most probably responsible for the SAW-induced lubrication. This explanation is also supported by vertical and longitudinal SAFM measurements, which show that, in areas where friction is completely suppressed, low frequency vertical cantilever oscillations can still be observed, whereas lateral or torsional oscillations are no longer excited.


Chemically vapor deposited Si nanowires nucleated by self-assembled Ti islands on patterned and unpatterned Si substrates

Physica E: Low-Dimensional Systems and Nanostructures 13 (2002) 995-998

TI Kamins, SS Williams, T Hesjedal, JS Harris

When Ti is deposited on Si in the 600-700 °C temperature range, the lattice mismatch between the Ti-containing deposit and the Si substrate causes TiSix nanoislands to form. The nanoislands grow when annealed at temperatures above 800 °C. When the nanoislands (either unannealed or annealed) are exposed to a Si-containing precursor gas, the Ti catalyzes the decomposition of the gas, allowing one-dimensional nanowires to grow. If oxide-patterned Si substrates are used, the Ti islands form selectively on the exposed Si and are preferentially positioned near the pattern edges. The subsequently grown Si nanowires are, therefore, positioned with respect to the larger lithographically formed pattern. Exposing the wires to an ion beam after deposition promotes the parallel alignment of nanowires. © 2002 Elsevier Science B.V. All rights reserved.


Structural and magnetic order in MnAs films grown by molecular beam epitaxy on GaAs for spin injection

MBE 2002 - 2002 12th International Conference on Molecular Beam Epitaxy (2002) 413-414

L Däweritz, M Kästner, T Hesjedal, T Plake, B Jenichen, KH Ploog

© 2002 IEEE. Epitaxial MnAs films on GaAs are of interest as ferromagnetic layers suitable for spin injection into semiconductors. Recent studies revealed that the phase transition near room temperature from the paramagnetic orthorhombic β phase above 40°C to the ferromagnetic hexagonal α phase below 40°C is of crucial importance for the structural and magnetic properties of the films. In this paper we report about ordered structures of the coexisting phases as a result of epitaxial strain and demonstrate the strong coupling between the magnetic order and the two-phase system.


Investigation of surface acoustic wave scattering effects

Proceedings of the IEEE Ultrasonics Symposium 1 (2001) 149-152

T Hesjedal, G Behme

We present measurements of surface acoustic wave (SAW) scattering from single dots, periodic and locally disturbed two-dimensional dot arrays. By using the scanning acoustic force microscope (SAFM), SAW fields of arbitrary polarization can be imaged with submicron spatial resolution and sub-Å wave amplitude sensitivity. The influence of a wavelength-sized single dot on SAW diffraction is studied. Forward- and back-scattered wave components can be imaged by insonifying the dot with a pump and a probe beam under different angles. SAW diffraction images of a regular dot array reveal a wavefield that is localized around the dots. In case of a disturbed scattering array, the localized SAW pattern vanishes in the vicinity of the distortion.


High-resolution imaging of a single circular surface acoustic wave source: Effects of crystal anisotropy

Applied Physics Letters 79 (2001) 1054-1056

T Hesjedal, G Behme

We present an experimental method for the high-resolution imaging of the excitation and propagation of surface acoustic waves (SAWs) on anisotropic piezoelectric substrates. By employing a scanning acoustic force microscope (SAFM), we are able to image acoustic waves that are excitable by a single circular electrode pair source through the mixing with well-defined reference plane waves. We show amplitude and phase images of the point-source wave field, containing the angular dependence of the phase velocity of these modes, as well as their electromechanical coupling strength. The SAFM allows easy access to acoustic material properties, which are important for the design of commercial SAW devices. © 2001 American Institute of Physics.


Investigation of crossed SAW fields by scanning acoustic force microscopy.

IEEE Trans Ultrason Ferroelectr Freq Control 48 (2001) 1132-1138

G Behme, T Hesjedal

We used multimode scanning acoustic force microscopy (SAFM) for studying noncollinearly propagating Rayleigh and Love wave fields. By analyzing torsion and bending movement of SAFM cantilever, normal and in-plane wave oscillation components are accessible. The SAFM principle is the down-conversion of surface oscillations into cantilever vibrations caused by the nonlinearity of the tip-sample interaction. Through mixing of complementary oscillation components, phase velocities of crossed Rayleigh waves on GaAs and crossed Rayleigh and Love waves on the layered system SiO2/ST-cut quartz were obtained simultaneously. Now, it is possible to investigate elastic properties of submicron areas through multimode SAFM measurements. Finally, we present mixing experiments of four SAWs on GaAs and discuss the various influences on the measured SAFM amplitude and phase contrast.


AFM observation of surface acoustic waves emitted from single symmetric SAW transducers.

IEEE Trans Ultrason Ferroelectr Freq Control 48 (2001) 641-642

T Hesjedal, G Behme

We report the first experimental observation of surface acoustic waves (SAWs) launched from a single symmetric SAW transducer, employing scanning acoustic force microscopy (SAFM). SAFM is a simple technique for the imaging of complex interdigital transducer (IDT) radiation patterns with nanometer lateral resolution. We demonstrate submicron lateral resolution and high sensitivity by investigating a single excitation element on a weakly coupling substrate (GaAs), visualizing the launched wave and second-order effects.


Influence of surface acoustic waves on lateral forces in scanning force microscopies

Journal of Applied Physics 89 (2001) 4850-4856

G Behme, T Hesjedal

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever's torsion and bending. The experiments show that friction can locally be suppressed by Rayleigh-type SAWs. Through the mapping of crossed standing wave fields, the wave amplitude dependence of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, friction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction effect. Thus, we conclude that the mechanical diode effect, i.e., the effective shift of the cantilever off of the oscillating surface, is responsible for the SAW-induced lubrication. This explanation is supported by vertical and lateral SAFM measurements: in areas with completely vanishing friction, low frequency vertical cantilever oscillations are still observable, whereas lateral (torsional) cantilever oscillations are no longer excited. Additionally, at very high Rayleigh wave amplitudes an effect of lateral force rectification was observed. It results in a scan direction-independent appearance of the LFM traces. © 2001 American Institute of Physics.


Study of elementary surface acoustic wave phenomena

Europhysics Letters 54 (2001) 154-160

T Hesjedal, G Behme

Many attempts have been made in acoustic microscopy to both achieve nanometer lateral resolution and sub-A wave amplitude detection. Employing a scanning acoustic force microscopy technique, acoustic wave properties of arbitrarily polarized modes can be measured with sub-wavelength resolution and high sensitivity. Surface acoustic wave fields of elementary model systems like a single scatterer and a single wave source are analysed in detail. We are able to observe radiation patterns, revealing the influence of the anisotropy of the GaAs substrate and the angular distribution of the piezoelectric coupling coefficient.


Reflection and mode conversion of surface acoustic waves studied by scanning acoustic force microscopy

Applied Physics A: Materials Science and Processing 72 (2001) 491-493

G Behme, T Hesjedal

We present measurements of the reflection and mode conversion of surface acoustic waves (SAWs) by scanning acoustic force microscopy (SAFM). The SAFM offers a unique combination of high lateral resolution and high sensitivity towards acoustic modes of all polarizations. Since a SAW mixing experiment of two waves can be performed even if the amplitude difference between both waves is 40 dB, wavefields of extremely small amplitudes can be investigated. Using SAFM, the reflection of SAWs from a metallic wedge is investigated with submicron lateral resolution. We are able to identify two reflected wave modes, a Love and a non-coupling Rayleigh mode, by measuring their phase velocities.


High-resolution imaging of surface acoustic wave scattering

Applied Physics Letters 78 (2001) 1948-1950

T Hesjedal, G Behme

We examine the scattering of surface acoustic waves (SAWs) by single dots, periodic and locally damped two-dimensional dot lattices. Employing the scanning acoustic force microscope, SAW fields are imaged with nanometer resolution. We study the influence of a roughly wavelength-sized single dot on SAW diffraction. In order to distinguish between forward- and backscattered components, we insonify the dot with the pump and probe beam under 0° and 90°. We furthermore analyse the SAW diffraction by a regular dot array. The wave field appears to be localized around the dots. Adding surface distortions, the regular SAW localization pattern brakes down in the vicinity of the distortion. © 2001 American Institute of Physics.


Ti-catalyzed Si nanowires by chemical vapor deposition: Microscopy and growth mechanisms

Journal of Applied Physics 89 (2001) 1008-1016

TI Kamins, RS Williams, DP Basile, T Hesjedal, JS Harris

Si nanowires grow rapidly by chemical vapor deposition on Ti-containing islands on Si surfaces when an abundant supply of Si-containing gaseous precursor is available. The density of wires is approximately the same as the density of the nucleating islands on the Si surface, although at least two different types of islands appear to correlate with very different wire growth rates. For the deposition conditions used, a minority of long, defect-free wires form, along with more numerous wires containing defects. Energy-dispersive x-ray spectroscopy shows that the Ti-containing nanoparticles remain at the tip of the growing wires. The estimated diffusion coefficient of Si in TiSi2 is consistent with the catalyzing nanoparticle remaining in the solid phase during nanowire growth. © 2001 American Institute of Physics.


MR-microscopy at 35 µm on a whole body MR-system: quality control by modulation transfer function and applications

Proc. Int. Soc. Mag. Reson. Med. 9 (2001) 929-

A Berg, HD Wanzenböck, T Hesjedal, E Moser


Chemically vapor deposited Si nanowires nucleated by self-assembled Ti islands on patterned and unpatterned Si substrates

Proceedings of MSS10 (2001)

TI Kamins, RS Williams, JS Harris, T Hesjedal


Study of the influence of surface acoustic waves on friction

Proceedings of the IEEE Ultrasonics Symposium 1 (2000) 599-602

T Hesjedal, G Behme

We present the study of the influence of surface acoustic waves (SAWs) on point-contact friction in scanning force microscopy. First, the effect of friction reduction due to Rayleigh-type SAWs was demonstrated. In order to visualize the dependence of this effect on the wave amplitude, we mapped standing wave fields in two dimensions. In order to detect and to distinguish between the influence of in-plane and vertical surface oscillation components on the cantilever's torsion and bending, we employed both lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM). We found that the friction reduction effect is only due to the vertical oscillation component. Moreover, as this effect doesn't appear for purely in-plane polarized Love waves, we conclude that the mechanical diode effect is most probably responsible for the SAW-induced lubrication.

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